This invention concerns injection molded closure caps and particularly such closure caps which address the problem of warping during post-molding curing.
In the interests of economy, injection molded plastic caps have been reduced in thickness and weight. A 110 mm cap (110-400), for example, can have a weight of less than about 18 grams, including the cap seal. One effect is that the top disk or panel becomes even more prone to warping during cooling and curing of the cap after molding, a process that can take about 24 hours. Warping can be induced by storing the just-molded closures in a container in random arrangement. This puts warping forces against the molded closures during curing, particularly those near the bottom of a bin or case. As a result, problems are encountered during automated assembly of the threaded closure cap onto a container.
A solution to this problem was devised by the assignee of the present invention, and has been used for several years. This solution has been to stack the caps coaxially, forming stacks or “logs” of caps by spinning each cap as it emerges from the mold, allowing them to “walk” along rotating rods to settle into a coaxially stacked log. In this way, all of the closures in a stack or log of caps are maintained in the proper shape during the curing period. Caps can be made lighter and thinner as a result of this log stacking process. Closure caps produced for such handling and stacking have included a nesting recess in the skirt of the closure, enabling the top of one cap to nest within the bottom edge of the skirt of a succeeding cap, resting on a ledge in the recess. Another benefit of stacking is compact storage, allowing more caps to a shipping case.
Although the stacking feature on the described caps, which included large 110 mm caps, worked well, the closures sometimes tended to cross-thread when screwed onto a container neck, especially in an assembly line capping operation in which containers were filled and closed. This caused an unacceptable rejection rate in the filling/assembly process. The configuration of the cylindrical recess for nesting the top of the succeeding cap tended to allow the cap to catch on the bottle finish and to become canted and this led to occasional cross-threading. This problem is related to the “S” dimension, which is defined as the dimension from the bottom of the cap's skirt up to the bottom of the thread start. In the case of the subject cap with the cross-threading problems, there were in essence two “S” dimensions: the distance from the skirt bottom up to the rim or ledge; and the distance from the ledge to the thread start. This simply provided too great an opportunity for canting and cross-threading, since the ledge at one side could catch on the bottle finish during cap assembly.